1. Field of the Invention
The present invention relates to an optical scanning apparatus used for an exposure apparatus in a laser printer, a laser facsimile apparatus, a digital copying machine, or the like to scan a laser beam.
2. Description of the Related Art
In general, the exposure optical systems of electrophotographic printers are roughly classified into two types: a system using a solid-state device such as an LED or a liquid crystal shutter; and a system using a scheme of scanning a laser beam.
In a system using the scheme of scanning a laser beam, optical aberrations such as main scanning/subscanning field curvatures, a scanning line curvature, an f-.theta. error, and the inclinations of reflection surfaces are generally compensated for in a purely optical manner. A structure obtained by combining a plurality of f-.theta. lenses and a cylindrical lens is used to compensate the inclinations of the reflection surfaces. In some case, in order to simplify the optical system, one surface of an f-.theta. lens is a toric surface to compensate the inclinations of the reflection surfaces.
An f-.theta. lens is, however, large in size and high in cost. For this reason, some known system is designed to perform optical aberration correction by using a combination of an elliptic cylindrical polygon mirror and a double aspherical surface correction lens without using any f-.theta. lenses.
In addition, Jpn. Pat. Appln. KOKAI Publication No. 2-131212 proposes a technique of electrically correcting an f-.theta. error of the optical aberrations and optically correcting other aberrations.
As shown in FIG. 17, a laser beam from a semiconductor laser 1 is collimated by a collimator lens 2. The laser beam is then transmitted through a cylindrical lens 3 to be incident on a polygon mirror 4. The polygon mirror 4 is rotated by a motor to deflect and scan the incident laser beam.
The deflected beam from the polygon mirror 4 is transmitted through a toroidal lens 5 and is reflected by reflecting mirrors 6 and 7 to be focused on a photosensitive drum 8. At the same time, the laser beam is scanned along a main scanning line 9 of the photosensitive drum 8.
In this case, the laser beam is scanned along the main scanning line 9 at different scanning speeds at a central portion and end portions of the line 9. That is, the scanning speed at end portions is higher than that at the central portion.
Owing to such a difference in scanning speed, if a laser beam is ON/OFF-controlled at a constant timing to expose a charge portion on the photosensitive drum 8, the pitch of dots formed by the ON/OFF operation of the laser beam becomes larger at the end portions than at the central portion.
In order to solve such a problem, in the invention disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-131212, the entire area of the main scanning lines 9 is divided into equal parts at the middle point of the line, and each half area is divided into seven blocks a to g.
As shown in FIG. 18, a reference clock having a frequency 10 times higher than that of a printing clock is used in such a manner that in each of the blocks a to g, the ratio of dots each formed by 10 reference pulses to dots each formed by nine reference pulses is changed. That is, the dot printing timing is quickened toward the end portions of the main scanning line 9 to make the dot pitches from the central portion to the end portions uniform macroscopically.
If, however, a uniform dot pitch is to be achieved by simply changing the ratio of dots each formed by 10 pulses to dots each formed by nine pulses, as in this official gazette, vertical moirs are produced between adjacent lines scanned by a laser beam, because dots each formed by 10 pulses and dots each formed by nine pulses, formed on adjacent lines, are aligned with each other.